This invention relates to a digital scan converter, and more particularly to techniques for updating display data in a digital scan converter with a selected characteristic in respect to peak detection capabilities in order to emulate the operation of an analog scan converter.
Electrostatic storage tubes have been widely used for scan conversion memory. Typical is the type H-1269A scan converter tube manufactured by Hughes Aircraft Comapny and used in a model 639 scan conversion memory also manufactured by Hughes Aircraft Corporation. That analog scan conversion memory has been used in such applications as ultra-sound imaging for medical diagnosis as well as many non-medical applications.
The key component of the analog scan conversion system is, of course, the scan converter tube which is essentially a cathode ray tube operated in a manner analogous to a conventional picture tube, except that the electron beam is directed toward a charge-storage target rather than a phosphor viewing screen. During the WRITE mode, the electron beam is driven in a vector scan mode corresponding to the signal source and is modulated in accordance with a video input signal so that local charges, proportional to beam intensity, are deposited on the storage target at discrete memory locations. In the READ mode, the beam (unmodulated) scans the storage target in a conventional interlaced raster mode. As the unmodulated beam scans, target output current varies with the charge pattern.
In the ultrasonic imaging application, as in other applications, it is necessary to process the video signal from a transducer before entering it into the scan conversion memory. The processing includes peak detection for each picture element (pixel) which, if displayed with other peak detected pixels, may present a rather "harsh" picture with significant noise. The scan converter tube does not store the peak detected pixels at full value but rather with a transfer function characteristic of the tube which is about 90% of the detected peak value on the first scan and about 100% on about the tenth scan over the same pixel. The result is inherent filtering of noise and softening of the picture displayed. This has been very helpful to medical doctors in ultrasonic imaging of body tissue for diagnostic analysis. It would therefore be important to the medical doctors skilled in the use of this diagnostic technique to always have a system with the same display characteristic. Therefore any digital scan converter should have the same transfer function characteristic as the analog scan converter tube for applications such as ultrasonic diagnostic analysis. However, for other applications it may be desirable to have some other transfer function, such as 70% on the first scan, an increase of 70% of the difference on each subsequent scan of greater value, and a decrease of 20% on each subsequent scan of lower value. And in some cases, it may be desirable to temporarily scan with a 100% transfer function for a "hard" picture, such as while trying to enhance some detail in the softened picture.